Semiconductor device and method of manufacturing the same

ABSTRACT

A semiconductor device can be manufactured which has a low resistance, and device characteristics of which do not vary. The semiconductor device includes a silicon layer, a gate dielectric film formed on the silicon layer, a gate electrode formed on the gate dielectric film and including a nitrided metal silicide layer which is partially crystallized, and source and drain regions formed in a surface region of the silicon layer at both sides of the gate electrode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2003-169700, filed on Jun. 13,2003, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method ofmanufacturing a semiconductor device.

2. Background Art

When a CMOS (Complementary Metal-Oxide-Semiconductor) device in thesubmicron (0.1 μm) gate-length generation is manufactured, it isvirtually impossible to use silicon (including an alloy with germanium,which will be referred to as “Si(Ge) gate” hereinafter), which hasconventionally been used as a material of a gate electrode, in the samemanner as before.

The first reason for this is that the resistivity of a Si(Ge) gate ishigh, i.e., a few hundreds μΩ·cm. When an actual layer thickness is 100nm, the sheet resistance value becomes a few tens Ω/square. It isexpected that in a semiconductor device of the submicron gate-lengthgeneration, an RC delay becomes evident with a resistivity of a gateelectrode of 5 Ω/square or more, thereby curbing the high-speedoperation of the device.

The second reason is the problem of Si(Ge) gate electrode depletion.This is a phenomenon in which a depletion layer having a limited lengthextends at the silicon gate electrode side of an interface between asilicon gate electrode and a gate dielectric film since the solubilitylimit of a dopant impurity added to silicon is about 1×10²⁰ cm⁻³.

The depletion layer substantially serves as a capacitance connected inseries to the gate dielectric film. Accordingly, this depletioncapacitance, which is about 0.3 nm when converted to a silicon oxidelayer, is added to the gate dielectric film. A gate dielectric film of afuture generation is required to have a gate capacitance of 1.5 nm orless calculated as the thickness of a silicon oxide layer. The additionof a capacitance of 0.3 nm caused by the gate depletion is deemed toserve as a factor that would strictly limit the formation of a thinnergate dielectric film.

The third reason is a problem in that an impurity such as boron, whichis added to decrease the resistance of the Si(Ge) gate, is thermallydiffused into a silicon substrate through the gate dielectric filmduring a high temperature heat treatment step in the manufacturing of anLSI. Such impurity can be a factor for variance in the threshold voltageof the FET, thereby considerably degrading the electric characteristicsof the device. Since an LSI in a future generation is required to have afar thinner gate dielectric film, the problem of thermal diffusion of animpurity added to a Si(Ge) gate to a silicon substrate is expected tobecome more serious in the future.

In order to solve the problems of Si(Ge) gates, high melting pointmetals such as molybdenum, tungsten, tantalum, etc., and nitridesthereof are used to form a gate electrode. This is known as metal gatetechnology.

The resistivity of a metal gate is lower than that of a Si(Ge) gate.Accordingly, the problem of the RC delay can be considerably alleviated.Furthermore, since the free-electron concentration within a metal gateis two or more orders higher than that of a Si(Ge) gate, and the widthof the gate depletion is decreased by one or more orders, the addedcapacitance, which is 0.3 nm (calculated as that of a silicon oxidelayer) in the case of a Si(Ge) gate, can be decreased to a level thatcan be ignored in the case of a metal gate. Moreover, since it is notnecessary to add an impurity to a metal gate in order to decrease theresistance, there is no problem of impurity penetration through a gatedielectric film. Thus, a metal gate is expected to solve the problems ofa Si(Ge) gate.

When a CMOS device including a metal gate is formed, the so-called dualphi (φ) metal gate technology is required, in which a metal materialhaving a work function of p⁺ silicon, and a metal material having a workfunction of n⁺ silicon are used to form a p-channel MOSFET and ann-channel MOSFET, respectively. Using this technology, the thresholdvoltages of the p-channel MOSFET and the n-channel MOSFET can becompletely controlled.

In the conventional art, TiN and TiAlN are proposed as a material for ametal gate of a p-channel MOSFET, and TaSiN is proposed as a materialfor a metal gate of an n-channel MOSFET (for example, D-G. Park, “IEDM”,Tech. Digest (2001) p. 671, and S. B. Samavedam, “VLSI”, Tech. Digest(2002) p. 24).

With such characteristics as having appropriate work functions, andbeing unmelted at a high temperature of about 900° C., thereby avoidingfailures at an interface with a gate dielectric film, these materialsare considered to be highly practical materials for forming a metalgate.

However, there are problems for these potentially practical materials.First, TiN and TiAlN are in a polycrystalline state. A gate electrode ina polycrystalline state would have the following problems.

The first problem is the mixing of an impurity into the lower part of agate electrode during an ion implantation step. As a conventional way ofmanufacturing an LSI, ions are implanted to a processed gate electrode,thereby forming a source and a drain in a self-aligned manner withrespect to the gate. However, when the gate electrode has apolycrystalline structure, accelerated ions pass through a grainboundary without scattering. Accordingly, the impurity isunintentionally introduced to a channel portion, thereby degradingcharacteristics of the device by varying the threshold voltage, etc.

The second problem is that since the grain size of the polycrystallinematerial forming the gate electrode is about the same as the devicesize, obviously a case may arise in which a gate electrode forming asingle FET includes only a few crystal grains. In such a case, it ishighly possible that variations in the number of crystal grains, thesize thereof, and the crystal orientation thereof in the respective FETsmay lead to variations in device characteristics.

On the other hand, TaSiN maintains an amorphous state even if it issubjected to high temperatures. This material can solve theaforementioned first and second problems involving polycrystallinematerial. However, there is a problem in that TaSiN in an amorphousstate has extremely high resistivity. Generally, the resistance of asilicide can be decreased only when it is crystallized. Thus, the reasonwhy the resistivity of TaSiN is high is this material is in an amorphousstate.

As described above, a conventional Si(Ge) gate has such problems as ahigh resistivity leading to an RC delay, thereby curbing the high-speedoperation of the device, the depletion of a Si(Ge) gate electrodeleading to a loss of gate dielectric film capacitance, and thethresholds voltage varying due to the penetration of an impurity fromthe silicon gate electrode into the gate dielectric film.

Metal gate materials conventionally used to solve the aforementionedproblems also have a dilemma in that ion penetration occurs with a lowresistance material which is in a polycrystalline state, thereby varyingthe device characteristics, while an amorphous material cannot decreasethe resistivity.

SUMMARY OF THE INVENTION

A semiconductor device according to a first aspect of the presentinvention includes: a silicon layer; a gate dielectric film formed onthe silicon layer; a gate electrode formed on the gate dielectric filmand including a nitrided metal silicide layer which is partiallycrystallized; and source and drain regions formed in a surface region ofthe silicon layer at both sides of the gate electrode.

A semiconductor device according to a second aspect of the presentinvention includes:

an n-channel MOSFET including: a p-type silicon layer formed on a firstregion of a semiconductor substrate; a first gate dielectric film formedon the p-type silicon layer; a first gate electrode formed on the firstgate dielectric film and including a nitrided metal silicide layer whichis partially crystallized; and n-type first source and drain regionsformed in a surface region of the p-type silicon layer at both sides ofthe first gate electrode, and

a p-channel MOSFET including: an n-type silicon layer formed on a secondregion of the semiconductor substrate; a second gate dielectric filmformed on the n-type silicon layer; a second gate electrode formed onthe second gate dielectric film and including a nitrided metal silicidelayer which is partially crystallized; and p-type second source anddrain regions formed in a surface region of the n-type silicon layer atboth sides of the second gate electrode.

A method of manufacturing a semiconductor device according to a thirdaspect of the present invention includes: forming gate dielectric filmsin a region of a semiconductor substrate where a p-channel MOSFET is tobe formed, and in a region of the semiconductor substrate where ann-channel MOSFET is to be formed; forming first gate electrode materiallayers of a partially crystallized nitrided metal silicide for forming ap-channel MOSFET on the gate dielectric films; forming a covering layerfor covering only the first gate electrode material layer in the regionwhere a p-channel MOSFET is to be formed; patterning the first gateelectrode material layer in the region where an n-channel MOSFET is tobe formed using the covering layer as a mask, leaving the first gateelectrode material layer only in the region where a p-channel MOSFET isto be formed; forming a second gate electrode material layer of apartially crystallized nitrided metal silicide for forming an n-channelMOSFET on the gate dielectric film in the region where an n-channelMOSFET is to be formed; and removing the covering layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic sectional view showing the structure of a metalgate electrode of a semiconductor device according to the firstembodiment of the present invention, and FIGS. 1B and 1C are schematicsectional views showing the structures of metal gates of conventionalsemiconductor devices.

FIG. 2 shows the differences in characteristics caused by thedifferences in structure between the metal gate electrode of thesemiconductor device of the first embodiment and that of conventionalsemiconductor devices.

FIGS. 3A to 3E show XRD spectrums obtained from the study of therelationship between the crystal volume ratio and the nitrogen contentof a nitrided metal silicide.

FIG. 4 shows the dependence of grain size of a nitrided metal silicideon nitrogen content.

FIGS. 5A to 5D are SEM pictures taken during the study of influence ofnitrogen content on the flatness of a surface of a nitride metalsilicide.

FIG. 6 shows an XRD spectrum obtained in the study of thecrystallization behavior of TaSiN.

FIG. 7 shows an XRD spectrum obtained in the study of the structure of anitrided metal silicide used in a semiconductor device of the firstembodiment of the present invention after it is subjected to a heattreatment.

FIG. 8 shows the dependence on nitrogen content of sheet resistance of anitrided metal silicide used to form a gate electrode of thesemiconductor device according to the first embodiment of the presentinvention.

FIG. 9 shows the range of nitrogen content in a gate electrode of thesemiconductor device of the first embodiment of the present invention,in which it is possible to obtain a desired effect.

FIG. 10 shows the dependence on the nitrogen content of the crystalvolume ratio of a nitrided metal silicide used to form a gate electrodeof the semiconductor device according to the first embodiment of thepresent invention.

FIG. 11 shows XPS spectrums obtained in the study of atomic bond stateafter a heat treatment is performed on a nitrided metal silicide used toform the gate electrode of the semiconductor device according to thefirst embodiment of the present invention.

FIG. 12 schematically shows the work functions that should be met byelectrode materials for forming CMOSFETs according to the firstembodiment of the present invention, and indicates relative suitablematerials.

FIG. 13 shows nitrided metal silicide materials used to form thesemiconductor device according to the first embodiment of the presentinvention, and their work functions.

FIG. 14 schematically shows the binding energy of photoelectrons ejectedfrom hafnium nitride silicide at the Fermi level.

FIGS. 15A and 15B show XPS spectrums obtained in the study of change inwork function of nitrided metal silicide used to form the semiconductordevice according to the first embodiment of the present invention,caused by the addition of nitrogen.

FIG. 16 shows the performance of a gate electrode formed of a nitridedmetal silicide used in the semiconductor device according to the firstembodiment of the present invention and the dependence thereof onnitrogen content.

FIG. 17 is a sectional view showing a CMOS device manufactured accordingto the second embodiment of the present invention.

FIG. 18 is a sectional view showing a step of a method of manufacturinga CMOS device according to the second embodiment of the presentinvention.

FIG. 19 is a sectional view showing a step of the method ofmanufacturing a CMOS device according to the second embodiment of thepresent invention.

FIG. 20 is a sectional view showing a step of the method ofmanufacturing a CMOS device according to the second embodiment of thepresent invention.

FIG. 21 is a sectional view showing a step of the method ofmanufacturing a CMOS device according to the second embodiment of thepresent invention.

FIG. 22 is a sectional view showing a step of the method ofmanufacturing a CMOS device according to the second embodiment of thepresent invention.

FIG. 23 is a sectional view showing a step of the method ofmanufacturing a CMOS device according to the second embodiment of thepresent invention.

FIG. 24 is a sectional view showing a step of the method ofmanufacturing a CMOS device according to the second embodiment of thepresent invention.

FIG. 25 is a sectional view showing a step of the method ofmanufacturing a CMOS device according to the second embodiment of thepresent invention.

FIG. 26 is a sectional view showing a step of the method ofmanufacturing a CMOS device according to the second embodiment of thepresent invention.

FIG. 27 is a sectional view showing a step of the method ofmanufacturing a CMOS device according to the second embodiment of thepresent invention.

FIG. 28 shows the result of an analysis of nitrogen content of a gateelectrode of a p-channel MOSFET manufactured according to the secondembodiment of the present invention, the analysis being performed by anEELS analysis method.

FIG. 29 shows the result of an analysis of nitrogen content of a gateelectrode of an n-channel MOSFET manufactured according to the secondembodiment of the present invention, the analysis being performed by anEELS analysis method.

FIG. 30 is a sectional view showing the structure of a CMOS devicemanufactured according to a first modification of the second embodimentof the present invention.

FIG. 31 is a sectional view showing the structure of a CMOS devicemanufactured according to a second modification of the second embodimentof the present invention.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. It should be notedthat the present invention is not limited to the following embodiments,and that additional modifications are possible.

(First Embodiment)

A semiconductor device according to the first embodiment of the presentinvention will be described below with reference to FIGS. 1 to 16.

The semiconductor device of this embodiment includes a MOSFET. The mostremarkable feature of the present invention lies in that the MOSFET hasa structure whereby the metal gate thereof is partially crystallized. Inthis specification, “partial crystallization” means that a layerincludes crystallized portions and uncrystallized portions, and that thecrystallized portions are substantially uniformly distributed in thelayer.

The partially crystallized layer can solve the dilemma of theconventional metal gate materials, i.e, a polycrystalline materialhaving a low resistance but having problems such as variance incharacteristics, and an amorphous material solving the problems of thepolycrystalline material but having a high resistance.

A metal gate electrode including a partially crystallized layer of thesemiconductor device of this embodiment can be manufactured by using anitrogen added metal silicide as a metal gate material.

FIG. 1A shows the characteristics of the metal gate structure of thesemiconductor device of this embodiment, FIGS. 1B and 1C show thecharacteristics of the metal gate structures of conventionalsemiconductor devices, and FIG. 2 shows the secondary characteristicsresulting from the respective structures.

As can be understood from FIG. 1A, in the metal gate of this embodiment,a nitrogen added metal silicide (silicide nitride) is partiallycrystallized. That is to say, the portions in which the metal silicideis crystallized (shown as hatched portions) and the portions in whichthe metal silicide is not crystallized are mixed, and the crystallizedportions are uniformly distributed. Accordingly, in this embodiment,grain boundaries do not continuously exist in the thickness direction ofthe metal gate, thereby solving the problem of ion penetration.Furthermore, since the size of crystal grains in the partialcrystallization is smaller than that of the FET, the problem ofvariations in characteristics of FETs can be solved. In contrast withthis, as shown in FIG. 1B, in the case where a polycrystalline materialis used to form the metal gate of a semiconductor device, asconventionally performed, the grain boundaries continuously exist in thethickness direction of the metal gate, thereby causing a problem of ionpenetration. In FIG. 1B, the crystal grains are marked by hatched lines.Moreover, in a metal gate having a polycrystalline structure, the numberand size of crystal grains differ FET by FET. Accordingly, each FET hasdifferent characteristics. Furthermore, as shown in FIG. 1C, in the casewhere an amorphous material is used to form the metal gate of asemiconductor device, as conventionally performed, the amorphous layerhas a high resistance since it is not crystallized.

However, in this embodiment, it is possible to decrease the resistivityof the entire layer since the crystallized portions decrease theresistivity. This embodiment has taken over the advantage of amorphouslayers, i.e., having a high spatial uniformity, while overcoming thedisadvantage of such layers, i.e., having a high resistance.

Hereinafter, hafnium will be used as an example of a material of a metalgate, and the concepts of this embodiment will be described based on theresults of an experiment.

FIGS. 3A to 3E show the results of an experiment using the XRD (X-RayDiffractometry), in which the changes in structure of a silicide layerwere observed when nitrogen was added to hafnium silicide several timesso as to change the nitrogen content in stages. FIG. 3A shows anexperimental result when the nitrogen content was 0 at. %; FIG. 3B showsan experimental result when the nitrogen content was 12 at. %; FIG. 3Cshows an experimental result when the nitrogen content was 17 at. %;FIG. 3D shows an experimental result when the nitrogen content was 29at. %; and FIG. 3E shows an experimental result when the nitrogencontent was 51 at. %. The measurement of the nitrogen content wasperformed using RBS (Rutherford Backscattering Spectrometry).

In FIGS. 3A to 3E, the horizontal axis represents the diffraction angle2θ between the diffracted x-ray and the surface of the specimen, and thevertical axis represents the crystal diffraction intensity of the x-ray.The arrows in FIGS. 3A to 3D show hafnium silicide (HfSi₂) crystals. Inthese experiments, a heat treatment was performed at a temperature of900° C. in order to crystallize the hafnium silicide thin film.

In the cases where the nitrogen content in the hafnium silicide layerwas 0 at. % and 12 at. %, steep peaks of hafnium silicide were observed(FIGS. 3A and 3B). In the case where the nitrogen content in the hafniumsilicide layer was 0 at. %, the hafnium silicide layer was 100%crystallized (FIG. 3A). This means that the volume ratio in the crystalregion was 100%. In the case where the nitrogen content in the hafniumsilicide layer was 12 at. %, the hafnium silicide layer was 100%crystallized (FIG. 3B). This means that the volume ratio in the crystalregion (hereinafter also referred to as “crystal volume ratio”) was100%. It can be said that this layer was in a polycrystalline state. Thevolume ratio in the crystal region was obtained by dividing the volumeof the crystal region by the sum of the volume of the crystal region andthe volume of the amorphous region. The measurement of the volume ratioof the crystal region was performed using XRD. In FIGS. 3A to 3E, themaximum values of the crystal diffraction intensity of x-ray in thespectrums show the volume ratio in the crystal region.

It was clarified that when the nitrogen content increased to 17 at. % ormore (FIG. 3C), the crystal diffraction intensity of the hafniumsilicide layer decreased. This means that the volume ratio of thecrystal contained in the layer decreased, thereby causing the partialcrystallization. In the case where the nitrogen content was 12 at. % orless (FIG. 3B), the layer was 100% crystallized, while in the case wherethe nitrogen content was 17 at. %, the crystal volume ratio was 30%(FIG. 3C) and in the case where the nitrogen content was 29 at. %, thecrystal volume ratio was 10% (FIG. 3D). Thus, as the nitrogen contentincreased, the crystal volume ratio decreased.

It was clarified that in the case where the nitrogen content was as highas 51 at. % (FIG. 3E), the hafnium silicide layer remained in anamorphous state after a heat treatment at a temperature of 900° C. Thismeans that the hafnium silicide layer is still an amorphous layer.

The results of this experiment clarified that the partially crystallizedmetal gate structure as in the case of this embodiment could be achievedonly by adding an appropriate amount of nitrogen to hafnium silicide.The mechanism of curbing the conversion into a polycrystalline state andadvancing the partial crystallization by using nitrogen in thisembodiment seems to be as follows.

In general, hafnium silicide contains embryos. When a heat treatment isperformed, hafnium atoms and silicon atoms move within the hafniumsilicide layer, resulting in that the embryos are supplied with suchatoms, thereby causing nucleus growth. Ultimately, a large crystal grainof hafnium silicide is formed. Since the generation of embryos occursspatially at several points, a plurality of hafnium silicide crystalgrains are located in the layer in a close-packed manner, therebycompleting the polycrystalline structure.

When nitrogen is added to the layer, the diffusion of atoms, which isnecessary to perform the hafnium silicide nucleus growth, is curbed,thereby curbing the growth of crystal grains. At a nitrogen contentwhich results in cessation of the crystal growth before the crystalgrains come into contact with each other, the silicide layer is not in apolycrystalline state, but is in a partially crystallized state. Whenthe nitrogen content is too high, the movement of atoms is completelyinterrupted, thereby inhibiting the growth of embryos, resulting in thatthe layer is in an amorphous state.

FIG. 4 shows an experimental result proving that the addition ofnitrogen inhibits the crystal nucleus growth. FIG. 4 shows the crystalgrain size Dhkl of silicide in the silicide layer obtained from thehalf-width of the crystal diffraction peak of the XRD spectrums shown inFIGS. 3A to 3E. In FIG. 4, the horizontal axis represents the nitrogencontent, and the vertical axis represents the silicide crystal grainsize Dhkl in the silicide layer. When the nitrogen content was 12 at. %or less, with which no partial crystallization occured, the crystalgrain size was constant. When the nitrogen content was 17 at. %, atwhich the partial crystallization started, the grain size decreased.When the nitrogen content was 29 at. %, at which the partialcrystallization advanced further, the grain size became extremely small.It can be understood from these results that the addition of nitrogeninhibits the nucleus growth of hafnium silicide, which becomes moreevident as the nitrogen content is increased.

It was understood from the experiments that the downsizing of thecrystal grains of hafnium silicide caused by the addition of nitrogenhas advantages from the viewpoints of the improvement of oxidationresistant characteristic and the improvement of flatness of the metalgate electrode. FIGS. 5A to 5D are SEM (Scanning Electron Microscope)images of the surface of nitrogen added hafnium silicide layers havingbeen subjected to a heat treatment at a temperature of 900° C. in anoxygen/nitrogen atmosphere. FIG. 5A shows an SEM image when the nitrogencontent was 0 at. %; FIG. 5B shows an SEM image when the nitrogencontent was 12 at. %; FIG. 5C shows an SEM image when the nitrogencontent was 17 at. %; and FIG. 5D shows an SEM image when the nitrogencontent was 29 at. %. When the nitrogen content was 12 at. % or less,the degree of surface roughness of the layer after a heat treatment at900° C. was extremely high (FIGS. 5A and 5B). The first reason for thiswould be the expansion of projections and depressions on the surface ofthe layer due to the crystal growth of silicide, and the second reasonwould be that the oxidation of silicide caused the roughness of thesurface.

When the nitrogen content was 17 at. % or more, the surface of silicideafter being subjected to a heat treatment was extremely flat (FIGS. 5Cand 5D). The first reason why such flatness could be achieved would bethe effect of the crystal grain growth being inhibited, thereby makingthe grain size very small, and the second reason would be the oxidationof the layer being inhibited by the addition of nitrogen. Theimprovement in oxidation resistant property expands the degree offreedom in the LSI manufacturing process. That is to say, it is possibleto set the oxygen control content to be high, thereby curbing theincrease in resistance of the gate electrode. Further, when the surfaceis flat, the later steps can be facilitated, thereby decreasingvariations in device characteristics.

In order to achieve the partially crystallized metal gate of thisembodiment, not only the range of nitrogen content but also the type ofmetal should be limited. As described previously, the limitation ofdiffusion of atoms using nitrogen plays an important role in theformation of a partially crystallized layer. Inherently, the diffusionof atoms does not occur easily with a high melting-point material. Whennitrogen is added to such a material, no diffusion of atoms occurs. As aresult, a high melting-point metal has a polycrystalline structure whenno nitrogen is added thereto. The addition of only a slight amount ofnitrogen makes the high melting-point metal convert to an amorphousstructure. FIG. 6 shows an experimental result of XRD obtained in thestudy of the crystallization behavior of TaSiN using a highmelting-point metal, Ta. When 30 at. % of nitrogen was added, no crystalpeak was observed after a heat treatment at 900° C. This means that thislayer was in an amorphous state. A similar experimental result isdisclosed in S. B. Samavedam, “VLSI”, Tech. Digest (2002), p. 24.

In the case of Hf, which has a lower melting point than Ta, a partiallycrystallized layer as shown in FIGS. 3A to 3E was achieved. FIG. 7 showsan experimental result of XRD using platinum, which has a lower metingpoint that Hf. It was understood from FIG. 7 that in this case, apartially crystallized nitrided metal silicide layer could be formed.

The structure of a metal silicide material having a melting point of1,500° C. or less is destroyed during the process of manufacturing anLSI. A metal silicide material having a melting point of 2,500° C. ormore converts to an amorphous state when nitrogen is added thereto. Sucha metal silicide material cannot be partially crystallized. Accordingly,in this embodiment, the metal material used to form the gate electrodeis limited to those having a melting point of 1,500° C. or more and2,500° C. or less.

FIG. 8 shows the experimental result of measuring the resistivity of ahafnium silicide layer after being subjected to the annealing at 900°C., when the nitrogen content was 0 at. %, 12 at. %, 17 at. %, 29 at. %,and 51 at. %, as in the case of FIGS. 3A to 3E. In the cases where thenitrogen content of the partially crystallized metal silicide was 17 at.% and 29 at. %, the resistivity was lower than 5 Ω/square, which is therequired performance of the layer. This means that this material canmeet the requirement. As can be understood from FIG. 8, when thenitrogen content was 30 at. %, the resistivity became the requiredvalue, i.e., 5 Ω/square.

The reason why the resistance is kept low when the crystal volume ratiois 10%, as in the case where the nitrogen content was 29 at. %, isdeemed to be that at least with this crystal volume ratio, theelectrical conduction of the layer is highly dependent on thecrystallized portions of the layer. When the crystal volume ratio isless than 10%, the resistance value no longer meets the requiredperformance conditions. Accordingly, the crystal volume ratio of thenitrided metal silicide layer used to form the metal gate of thisembodiment is limited to 10% or more. From the viewpoint of the decreasein the resistance value, there is no upper limit in crystal volumeratio. However, the lower limit of the nitrogen content for achievingboth causing partial crystallization and avoiding such problems as thepenetration of ions and the variations in device characteristics, i.e.,15 at. %, determines the upper limit of the crystal volume ratio to be58%.

Next, the reason why the range of nitrogen content to obtain the effectsof this embodiment is 15 at. % to 30 at. % will be described withreference to FIG. 9. With respect to the upper limit of the nitrogencontent, since the sheet resistance becomes higher than the requiredvalue when the nitrogen content is 30 at. % or more (FIG. 8), thenitrogen content in the nitrided metal silicide in the metal gate ofthis embodiment should be limited to 30 at. % or less, as shown in FIG.9. With respect to the lower limit of the nitrogen content, as mentionedin the descriptions of the XRD experimental result shown in FIGS. 3A to3E, no partially crystallized layer is formed when the nitrogen contentis at 12 at. %, but a partially crystallized layer can be formed whenthe nitrogen content is 17 at. %. From an experiment in which thenitrogen content was changed in a precise fashion, it has become knownthat a partially crystallized layer can be formed only when the nitrogencontent is 15 at. % or more. Therefore, in this embodiment, the nitrogencontent of the nitrided metal silicide to form the metal gate should belimited to be 15 at. % or more, as shown in FIG. 9.

Thus, the range of the nitrogen content to obtain the effects of thisembodiment is 15 at. % to 30 at. %. As can be understood from FIG. 10,it is preferable that the crystal volume ratio of the nitrided metalsilicide in this range of nitrogen content be 10% to 58%. FIG. 10 plotsthe values of the crystal volume ratio (100%, 100%, 30%, 10% and 0%)when the nitrogen content in the metal silicide layer shown in FIG. 3 is0 at. %, 12 at. %, 17 at. %, 29 at. %, and 51 at. %, respectively.

Getting back to FIG. 8, when the nitrogen content was 50 at. % or more,the resistance rapidly increased by two orders or more. The reason forthis is deemed to be attributable to the layer structure becoming anamorphous state, as previously described. An experiment was performed tostudy other factors of the rapid increase by observing the changes fromthe viewpoint of the atomic bond using XPS (X-ray PhotoelectronSpectroscopy). FIG. 11 shows the result of this experiment, i.e., theatomic bond states of hafnium nitride silicide (HfSiN) after beingsubjected to a heat treatment at 900° C. It was clarified that when thenitrogen content was 51 at. %, there was only a bonding staterepresenting hafnium nitride within the layer. Since the surface ofHfSiN was slightly oxidized in this experiment, the peak representinghafnium oxide was superimposed on the peak representing hafnium nitridethereby making the peak rather broad. When the nitrogen content was 29at. %, the peaks representing hafnium silicide, in addition to the peaksrepresenting hafnium nitride, as indicated by arrows, were observed. Itwas clarified that the reason why a rapid increase in the sheetresistance value occurred when the nitrogen content was 51 at. % wasthat there was no hafnium silicide bond in the layer.

FIG. 11 also shows the bonding state of HfSiN when the nitrogen contentwas 12 at. %. Also in this case, there are peaks representing hafniumsilicide, indicating that the lowness of the sheet resistance value ofHfSiN at a lower nitrogen content is attributed to this atomic bond.However, no peak representing hafnium nitride was observed for the HfSiNlayer having a nitride content of 12 at. %, but only peaks representinghafnium oxide, generated by the surface oxidation of HfSiN, wereobserved. It seems that at this nitrogen content, nitrogen ispreferentially bonded to silicon, and that as a result, no hafniumnitride is formed. Since hafnium nitride is less prone to being oxidizedthan hafnium silicide, the reason why the oxidation resistant propertyof HfSiN layer at the nitrogen content of 12 at. % degraded is deemed tobe that a sufficient amount of hafnium nitride was not formed.

The nitrided metal silicide used to form the metal gate of thesemiconductor device according to this embodiment is effective to form agate electrode of a CMOS device. As shown in FIG. 12, it is not possibleto control the threshold voltage to be within an effective range if anelectrode material having a work function near that of n⁺polycrystalline silicon is used with respect to an n-channel MOSFET, oran electrode material having a work function near that of p⁺polycrystalline silicon is used with respect to a p-channel MOSFET, asshown in FIG. 12. In this embodiment, it is possible to achieve the sameperformance as a conventional polycrystalline silicon gate electrode byusing a nitrided metal silicide containing Ti, Zr, or Hf for a gateelectrode of an n-channel MOSFET, or using a nitrided metal silicidecontaining Pt, Pd, or Ir for a gate electrode of a p-channel MOSFET. Thereason for this is that the respective silicides have work functionsnear those of n⁺ polycrystalline silicon and p⁺ polycrystalline silicon,as shown in FIG. 13.

It is known that when a metal silicide is nitrided, the work functionthereof does not change considerably. Therefore, it is deemed that FIG.13 can apply to a nitrided metal silicide. FIGS. 14 to 15B show theproofs thereof. FIG. 14 schematically shows the binding energy ofphotoelectrons ejected from hafnium nitride silicide at the Fermi level.FIGS. 15A and 15B show experimental results of the study of whether thebinding energy changes in accordance with the changes in nitrogencontent in a hafnium nitride silicide layer formed by the use of XPS.FIG. 15A shows the case where the nitrogen content N was 0 at. %, andFIG. 15B shows the case where the nitrogen content N was 29 at. %.

As shown in FIG. 14, the work function φ_(m) is determined by the energyat the Fermi level Ef. Accordingly, it is possible to estimate therelative change of work function of different specimens in thisexperiment. As shown in FIGS. 15A and 15B, the binding energy of a Fermilevel electron of hafnium silicide and a nitrided metal silicide (at thenitride content of 29 at %) were −1.51 eV and −1.63 eV, respectively.From this experimental result, it was revealed that there is nomeaningful relative difference therebetween, i.e., a nitrided metalsilicide has a work function similar to a metal silicide.

FIG. 16 shows the dependence of gate electrode performance on nitrogencontent in the case where hafnium is used in a nitrogen added metalsilicide used to form the metal gate of the semiconductor of thisembodiment. As can be understood from FIG. 16 and the previousdescriptions, the range of nitrogen content in which the partiallycrystallized metal gate of this embodiment is superior in performanceand can show more advantageous effects than a conventional device is 15at. % or more and 30 at. % or less.

As described above, according to this embodiment, it is possible tomanufacture a semiconductor device which has a lower resistance, thedevice characteristics of which do not vary.

(Second Embodiment)

Next, a method of manufacturing a semiconductor device according to thesecond embodiment of the present invention will be described withreference to FIGS. 17 to 29. The semiconductor device manufactured bythis method is a CMOS device, in which metal gates of the firstembodiment are used to form the gate electrodes of an n-channel MOSFETand a p-channel MOSFET.

FIG. 17 is a sectional view showing the structure of a CMOS devicemanufactured by the manufacturing method of this embodiment. In thisCMOS device, a p-channel MOSFET 1 and an n-channel MOSFET 2 are formedon the same substrate. As shown in FIG. 17, the p-channel MOSFET 1 andthe n-channel MOSFET 2 are formed on the silicon substrate 4, andisolated by an element isolation region 6 having a shallow trenchstructure.

In a region of the silicon substrate 4 where the p-channel MOSFET 1 isformed, an N silicon well 3 is formed, while in a region where then-channel MOSFET 2 is formed, a P silicon well 5 is formed.

The p-channel MOSFET 1 includes a laminated structure (MIS structure)having a gate dielectric film 7 formed on the N silicon well 3 and ap-channel nitrided metal silicide gate electrode 8. The p-channelnitrided metal silicide gate electrode 8 is formed by using any of themetals Pt, Pd, and Ir. At the side of this laminated structure, a gatesidewall 15 is formed.

Deep p⁺ impurity diffusion layers 13 and shallow p⁺ impurity diffusionlayers 14 are formed in the N silicon well 3 at both sides of the gatedielectric film 7. These layers serve as a source and a drain. Aself-aligned silicide (salicide) layer 11 is formed on the deep p⁺impurity diffusion layers 13.

In a region where the n-channel MOSFET 2 is formed, the P silicon well 5is formed. The n-channel MOSFET 2 includes a gate dielectric film 7formed on the P silicon well 5, and an n-channel nitrided metal silicidegate electrode 12 formed by using any of the metals Ti, Zr, and Hf. Atthe side of this laminated structure, a gate sidewall 15 is formed.

Deep n⁺ impurity diffusion layers 16 and shallow n⁺ impurity diffusionlayers 17 are formed in the P silicon well 5 at both sides of the gatedielectric film 7. These layers serve as a source and a drain. Asalicide layer 11 is formed on the deep n⁺ impurity diffusion layers 16.

A method of manufacturing the CMOS device shown in FIG. 17 will bedescribed below with reference to FIGS. 18 to 27.

First, as shown in FIG. 18, the element isolation region 6 having ashallow trench structure is formed on the silicon substrate 4. After theN silicon well 3 and the P silicon well 5 are formed, the gatedielectric film 7, which is any of a silicon oxide (SiO₂) layer, asilicon oxynitride (SiON) layer, a metal oxide layer, and a metalsilicate layer, is formed. In the case of a metal oxide layer, at leastone metal selected from the group of zirconium, hafnium, titanium,tantalum, aluminum, and a rare-earth element such as yttrium, lanthanum,cerium, etc., can be used. It is preferable that a CVD method be used toform such a layer since the step coverage property is high, and thedamage to the silicon layer is slight. However, a sputtering method oran MBE method can also be used to obtain the effects of the presentinvention.

Next, as shown in FIG. 19, the p-channel nitrided metal silicide gateelectrode 8 is deposited on the entire surface of the gate dielectricfilm 7. For example, a PtSiN layer having a thickness of 100 nm can bedeposited. The deposition of the PtSiN layer can be formed by performingsimultaneous sputtering from a Pt target and a Si target. It is possibleto set the nitrogen content in the PtSiN layer to be about 20% by usingAr/N₂ mixture gas at the time of the sputtering and by setting the Ar/N₂gas flow ratio to be about 9 to 1. In this embodiment, the Ar flow rateis set to be 89 sccm, and the N₂ flow rate is set to be 9 sccm, and thesputtering power is set to be 100 W for Pt, and 250 W for Si, therebydepositing a PtSiN layer having a Pt/Si ratio of 1 to 1, and a nitrogencontent of 20 at. %. It is possible to adjust the Pt:Si ratio byadjusting the sputtering power. In this embodiment, it is preferablethat the Pt:Si ratio be about 1:1 to 3:5, since with this ratio, thecomposition of a Pt silicide in PtSiN becomes either PtSi or Pt₂Si,thereby decreasing the resistance further. The materials of thep-channel nitrided metal silicide that can be used in this invention,i.e., Pt, Pd, and Ir, are classified as noble metals in the periodictable of elements, and have substantially the same solid-stateproperties from a chemical viewpoint. Accordingly, it is possible toform the p-channel nitrided metal silicide gate electrode 8 by using Pdand Ir in a manner similar to that described above.

Next, as shown in FIG. 20, a mask layer 19 for covering only thep-channel MOSFET 1 is formed. In this embodiment, a Si₃N₄ layer 19 isformed by first depositing the Si₃N₄ layer 19 on the entire surface ofthe structure shown in FIG. 19 by using a CVD method, etc., forming aresist mask (not shown in the drawing) on the p-channel MOSFET 1 by anormal lithography technique, and then removing the portion of the Si₃N₄layer 19 that is not covered by the resist mask by heated phosphoricacid solution, thereby obtaining the structure shown in FIG. 20.

Next, the portion of the PtSiN layer 8 that is not covered by the Si₃N₄mask layer 19 is removed. In this step, first the silicon oxide layerformed at the uppermost portion of the PtSiN layer 8 is removed by HFsolution, and then, the PtSiN layer 8 is removed by aqua regiatreatment, thereby obtaining the structure shown in FIG. 21. At thistime, the gate dielectric film 7 deposited on the portion of then-channel MOSFET 2 is not etched.

Then, as shown in FIG. 22, the n-channel nitrided metal silicide gateelectrode 12 is formed on the entire surface. For example, a HfSiN layerhaving a thickness of 100 nm is formed in this embodiment by performinga simultaneous sputtering from a Hf target and a Si target. It ispossible to set the nitrogen content in the HfSiN layer to be about 20%by using Ar/N₂ mixture gas at the time of the sputtering and setting theAr/N₂ gas flow ratio to be about 20 to 1. In this embodiment, the Arflow rate is set to be 80 sccm, and the N₂ flow rate is set to be 4sccm, and the sputtering power is set to be 100 W for Hf, and 100 W forSi, thereby depositing a HfSiN layer having a Hf/Si ratio of 1 to 1, anda nitrogen content of 20 at. %. It is possible to adjust the Hf:Si ratioby adjusting the sputtering power. In this embodiment, it is preferablethat the Hf:Si ratio be about 1:1 to 3:5, since with this ratio, thecomposition of a Hf silicide in HfSiN becomes either HfSi or HfSi₂,thereby decreasing the resistance further. The materials of then-channel nitrided metal silicide that can be used in this invention,i.e., Ti, Zr, and Hf, are related elements in the periodic table ofelements, and have substantially the same solid-state properties from achemical viewpoint. Accordingly, it is possible to form the n-channelnitrided metal silicide gate electrode 12 by using Ti and Zr in a mannersimilar to that described above.

Next, the Si₃N₄ layer 19 is removed by using heated phosphorous acidsolution, thereby obtaining the structure shown in FIG. 23. The HfSiNlayer 12 deposited on the Si₃N₄ layer 19 is removed during the removingof the Si₃N₄ layer 19 due to the lift-off effect. In order to performthe lift-off of the HfSiN layer 12 on the Si₃N₄ layer 19 in a securedmanner, it is preferable that the thickness of the Si₃N₄ layer 19 bemore than three times the thickness of the HfSiN layer 12. In thisembodiment, the thickness is set to be 300 nm.

Next, as shown in FIG. 24, the shaping of the gate electrode portion isperformed by using a normal photolithography technique and etchingtechnique. It is preferable that the processing of the gate electrodeand the gate dielectric film be performed by using a physical etchingtechnique such as the Ar ion milling. The reason for this is that sincethe p-channel nitrided metal silicide gate electrode 8 and the n-channelnitrided metal silicide gate electrode 12 formed of different materialsare processed at the same time, a chemical etching mechanism may causean etching rate error, thereby changing the shape of the p-channelMOSFET and the n-channel MOSFET obtained thereby.

Then, ion implantation is performed in a conventional manner, therebyforming the shallow p⁺ impurity diffusion layers 14 and the shallow n⁺impurity diffusion layers 17 as shown in FIG. 25. In this step, the gateelectrodes serve as masks during the ion implantation. It has beenobserved that since the gate electrodes 8 and 12 of this embodiment arepartially crystallized, no accelerated ion reaches the Si channel regionunder the gate electrodes 8 and 12. Of course, during the ionimplantation to form the p-channel MOSFET, the n-channel MOSFET iscovered by a resist, and during the ion implantation to form then-channel MOSFET, the p-channel MOSFET region is covered by a resist.

Thereafter, as shown in FIG. 26, the gate sidewall 15 is formed in awell-known manner. Subsequently, the deep p⁺ impurity diffusion layer 13and the deep n⁺ impurity diffusion layer 16 are formed by performing anactivation heat treatment at a temperature of 900° C. This heattreatment is performed in atmospheric pressure nitrogen with an oxygenpartial pressure of 10⁻³ Torr or more. After this step, the surface ofeach nitrogen added metal silicide layer is oxidized to about 1 nm indepth. As the result, the surface is covered by an SiON layer (not shownin the drawing). In this step, during the ion implantation to form thep-channel MOSFET, the n-channel MOSFET is covered by a resist, andduring the ion implantation to form the n-channel MOSFET, the p-channelMOSFET region is covered by a resist.

Then, as shown in FIG. 27, the salicide layer 11 is formed on each ofthe p⁺ impurity diffusion layers 13 and n⁺ impurity diffusion layers 16.In this embodiment, first a Co/Ti/TiN deposition is performed, then a Cosilicide is formed by a heat treatment performed in a conventionalmanner, and then the Ti/TiN layer is removed by a mixed acid solutioncontaining H₂SO₄ and H₂O₂. Since the surfaces of the p-channel nitridedmetal silicide gate electrode 8 and the n-channel nitrided metalsilicide gate electrode 12 are covered by thin SiON layers, which areformed in the previous step, the gate electrodes are not eroded by themixed acid solution containing H₂SO₄ and H₂O₂ used to remove the Ti/TiNlayer. A usual silicide material may be entirely oxidized by a heattreatment under an oxygen atmosphere at a temperature of 900° C.However, in the case of the nitrogen added metal silicide layers of thisembodiment, only part of the surfaces of the layers is oxidized. Thereason for this is that the diffusion of oxygen is curbed at the SiONlayer, thereby stopping the oxidation at a certain point. Thereafter, anormal device flattening step is performed, thereby completing themanufacturing of the CMOSFET as shown in FIG. 17.

In the CMOS device of this embodiment thus manufactured, the fact thatnitrogen is contained in the gate electrode portion was confirmed in thefollowing manner. A section of the CMOS device as shown in FIG. 17 wasexposed by specimen processing using an FIB (Focused Ion Beam) method,and a composition analysis of the gate electrode portion was performedby EELS (Electron Energy Loss Spectroscopy) using a TEM (TransmissionElectron Microscopy) apparatus. The reason why EELS was employed is thatthis method is suitable for the analysis of light elements such asnitrogen. As shown in FIG. 28, the p-channel gate electrode 8 formed ofPtSiN contained 15 at. % of nitrogen. Furthermore, as shown in FIG. 29,the n-channel gate electrode 12 formed of HfSiN contained 25 at. % ofnitrogen. Although signals representing oxygen are observed in FIGS. 28and 29, the reason for the generation of oxygen is the oxidation of thesurface of the section of the gate electrodes during the formation ofspecimens for EELS. The gate electrodes contain no oxygen.

As described above, according to this embodiment, it is possible tomanufacture a semiconductor device which has a lower resistance, thedevice characteristics of which do not vary.

Before the patterning of the gate electrodes shown in FIG. 24, it ispossible to form a layer of Si or SiGe on the nitrided metal silicidegate electrodes 8 and 12. With this step, it is possible to improve theoxidation resistant property in the later steps and the process matchingcharacteristics of the gate electrodes without degrading thecharacteristics of the partially crystallized nitrided metal silicidegate electrodes. The reason for this is that Si or SiGe is highlyresistant to an oxidation atmosphere as compared to a nitrided metalsilicide. Accordingly, in this case, the CMOS device manufacturedincludes a layer 30 of Si or SiGe formed on the nitrided metal silicidegate electrodes 8 and 12, as shown in FIG. 30.

Furthermore, as shown in FIG. 31, it is possible to form a layer 35 of ametal, a metal silicide, a metal boride, a metal carbide, or a metalnitride, on the nitrided metal silicide gate electrodes 8 and 12. Inthis case, since a metal, a metal silicide, a metal boride, a metalcarbide, or a metal nitride has a lower resistivity than a nitridedmetal silicide, it is possible to decrease the resistance of the gateelectrodes without degrading the characteristics of the partiallycrystallized nitrided metal silicide gate electrodes.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcepts as defined by the appended claims and their equivalents.

1. A semiconductor device comprising: a silicon layer; a gate dielectricfilm formed on the silicon layer; a gate electrode formed on the gatedielectric film and including a nitrided metal silicide layer which ispartially crystallized; and source and drain regions formed in a surfaceregion of the silicon layer at both sides of the gate electrode.
 2. Thesemiconductor device according to claim 1, wherein a metal contained inthe nitrided metal silicide layer is selected from the group consistingof titanium, zirconium, hafnium, platinum, palladium, and iridium. 3.The semiconductor device according to claim 2, wherein: when the siliconlayer is an n-type channel, the nitrided metal silicide layer containsat least one metal selected from the group consisting of titanium,zirconium, and hafnium; and when the silicon layer is a p-type channel,the nitrided metal silicide layer contains at least one metal selectedfrom the group consisting of platinum, palladium, and iridium.
 4. Thesemiconductor device according to claim 1, wherein a melting point of ametal contained in the nitrided metal silicide layer is 1,500° C. ormore and 2,500° C. or less.
 5. The semiconductor device according toclaim 1, wherein a nitrogen content of the nitrided metal silicide layeris 15 at. % or more and 30 at. % or less.
 6. The semiconductor deviceaccording to claim 5, wherein the nitrogen content of the nitrided metalsilicide is 17 at. % or more and 29 at. % or less.
 7. The semiconductordevice according to claim 1, wherein a volume ratio of a crystal regionof the nitrided metal silicide layer is 10% or more and 58% or less. 8.The semiconductor device according to claim 1, wherein the gateelectrode includes a layer of Si or SiGe formed on the nitrided metalsilicide layer.
 9. The semiconductor device according to claim 1,wherein the gate electrode includes a layer of a material selected fromthe group consisting of a metal, a metal silicide, a metal boride, ametal carbide, and a metal nitride, formed on the nitrided metalsilicide layer.
 10. A semiconductor device comprising: an n-channelMOSFET including: a p-type silicon layer formed on a first region of asemiconductor substrate; a first gate dielectric film formed on thep-type silicon layer; a first gate electrode formed on the first gatedielectric film and including a nitrided metal silicide layer which ispartially crystallized; and n-type first source and drain regions formedin a surface region of the p-type silicon layer at both sides of thefirst gate electrode, and a p-channel MOSFET including: an n-typesilicon layer formed on a second region of the semiconductor substrate;a second gate dielectric film formed on the n-type silicon layer; asecond gate electrode formed on the second gate dielectric film andincluding a nitrided metal silicide layer which is partiallycrystallized; and p-type second source and drain regions formed in asurface region of the n-type silicon layer at both sides of the secondgate electrode.
 11. The semiconductor device according to claim 10,wherein: the nitrided metal silicide layer included in the first gateelectrode contains at least one metal selected from the group consistingof titanium, zirconium, and hafnium; and the nitrided metal silicidelayer included in the second gate electrode contains at least one metalselected from the group consisting of platinum, palladium, and iridium.12. The semiconductor device according to claim 10, wherein a meltingpoint of a metal contained in the nitrided metal silicide layers of thefirst and second gate electrodes is 1,500° C. or more and 2,500° C. orless.
 13. The semiconductor device according to claim 10, wherein anitrogen content of the nitrided metal silicide layers of the first andsecond gate electrodes is 15 at. % or more and 30 at. % or less.
 14. Thesemiconductor device according to claim 10, wherein a volume ratio of acrystal region of the nitrided metal silicide layers of the first andsecond electrodes is 10% or more and 58% or less.
 15. The semiconductordevice according to claim 10, wherein each of the first and second gateelectrodes includes a layer of Si or SiGe formed on the nitrided metalsilicide layer.
 16. The semiconductor device according to claim 10,wherein each of the first and second gate electrodes includes a layer ofa material selected from the group consisting of a metal, a metalsilicide, a metal boride, a metal carbide, and a metal nitride, formedon the nitrided metal silicide layer.